Aromatic polycarbonates have excellent mechanical characteristics such as impact resistance as well as heat resistance and transparency and have been employed as engineering plastics in broad range fields, such as bottles for carbonated beverages, electronic bases (CD bases), transfer belts, etc.
Industrially established processes for producing an aromatic polycarbonate include the so-called phosgene process comprising reacting an aromatic diol, e.g., bisphenol, and phosgene by interfacial polycondensation. The phosgene process is advantageous because the resulting aromatic polycarbonate has a heat resistance as high as about 500.degree. C. in terms of a heating temperature for 5% weight loss (Td5%) as hereinafter described.
However, the phosgene process which is currently carried out on an industrial scale has many disadvantages, such as high toxicity of phosgene, the necessity of handling of quantities of sodium chloride as a by-product, and fears of environmental pollution by methylene chloride which is usually used as a reaction solvent.
The so-called melt process or non-phosgene process is also well known and it consists of an interesterification reaction between an aromatic diol compound and a carbonic acid diester. The non-phosgene process is free of the above-mentioned problems associated with the phosgene process and also is more economical.
However, it is generally considered difficult to selectively obtain an aromatic polycarbonate having a high molecular weight and without any hydroxyl structure at the terminals thereof from, for example, bisphenol A and diphenyl carbonate, using the non-phosgene process. The presence of hydroxyl structure-terminated polymer molecules in an aromatic polycarbonate is considered as to be one of the reasons for a reduction in heat resistance. That is, as compared with the aromatic polycarbonate prepared by the phosgene process in which a hydroxyl terminal structure can be blocked out with ease, an aromatic polycarbonate prepared by the non-phosgene process is less heat resistant with a Td5% of 445.degree. C. or lower.
Because molding of aromatic polycarbonates should be conducted at high temperatures of around 320.degree. C. in order to lower melt viscosity thereof, low heat resistance of polycarbonates gives rise to problems such as cleavage of the polymer main chain, coloration, and reduction in mechanical strength. In particular, a high temperature is needed in molding to obtain thin-walled articles such as containers having a wall thickness of from 0.3 to 0.6 mm or articles with complicated shapes. Therefore, in order that an aromatic polycarbonate obtained by the non-phosgene process may be put to practical use, an improvement in heat resistance has been a keen demand.